Fossil Focus: Eggs, nests and dinosaur reproduction

Introduction

Of all the dinosaur fossils, skeletons are most fascinating to the public, because they represent real evidence of dinosaurs’ existence. When the study of skeletons is combined with information from fossilized footprints (which show how and how fast dinosaurs walked), dinosaurs seem to come to life: the body seems to move and interact with the substrate. But in real life, dinosaurs lived in similar ways to modern animals, and by asking the proper questions of some singular fossils, researchers can find out about their biology, such as their feeding strategies, growth and reproduction. Fossil eggs and nests are the only evidence about the reproductive biology of dinosaurs.

The study of oological fossils

Eggs and nests are called indirect fossils because they are not real (direct) parts of the organism that produced them. However, only the nests must be regarded as trace fossils. Eggs are not considered true trace fossils, because they formed inside the animal and did not result from the interaction of the animal with the substrate. However, both eggs and nests yield very significant information about the reproduction of dinosaurs, including data on the method of incubation, parental care and nesting and laying strategies. For instance, by analysing the water vapour conductance of the eggshells, researchers can infer whether the eggs were incubated underground, covered by sediment or vegetation mounds, or brooded by an adult sitting on them.

Eggshells

The study of eggshells, like that of other fossils, is based on the description of morphological characters (form and shape) and direct measurements. Close analysis looks at the eggshell microstructure, describing the minute characteristics of the calcified shell. It usually uses two main techniques: thin sections of the shells that are observed under a petrographic microscope, and analyses under a scanning electron microscope (SEM; Fig. 1). Thin sections are literally very thin slivers of the shell, which can be taken vertically or horizontally, and are mounted on glass slides so that researchers can look at features such as the shape and size of shell units (the basic microsctructural unit in which the eggshell grows), growth lines, respiratory channels or pores, and spherulites (the point where shell units start growing). SEM provides very detailed views of the surface ornamentation, pores and calcite crystals, among other features. From these one can identify the basic structural category of a particular eggshell, which helps with the study of the ‘parataxonomy’ or ‘ootaxonomy’ (a system of classification based on the binominal system used for living organisms). Among the amniotes (egg-laying animals) that produce carbonate shells, palaeoologists have distinguished six basic structural types: geckonoid, testudinoid, crocodiloid, spherulitic, prismatic and ornithoid. Three of these have been identified in dinosaurs.

Figure 1 – (A) Petrographic microscope image and (B) Scanning electron microscope image of Megaloolithus eggshells, an oogenus referred to sauropod dinosaurs, from the Upper Cretaceous period (99.6 million to 65.5 million years ago) of Catalonia. Both images are in radial view.

Eggs

So far, palaeontologists been able to attribute eggs to less than a dozen particular dinosaur groups, based on the embryos found inside the eggs. Computed tomography (CT) scans of entire eggs help with this, and also provide data about the spatial relationship between embryonic remains, the infilling sediment and the inner eggshell fragments. This may confirm how and in what environment the baby dinosaur hatched from the egg. When no embryonic remains are found inside the egg, palaeontologists must classify it using external, characters. Dinosaur eggs show a wide variety of shapes, surface ornamentation and pore distributions. Typically, theropods produced ellipsoidal (oval) eggs with a smooth surface or faint striations or ridges following the long axis of the egg. Herbivorous dinosaurs — sauropods and ornithopods — laid eggs more spherical in shape, with surface ornamentation consisting of dense and compacted nodes or a network of nodes and irregular ridges (Fig. 2).

Figure 2 — Scanning electron microscope image of the outer surface ornamentation in a Spheroolithus eggshell, an oogenus referred to hadrosaur dinosaurs, from the Upper Cretaceous of Catalonia. See the network pattern made of irregular nodes and ridges.

Egg shape is probably regulated by the resistance that the egg experiences when passing down the ‘oviducts’, or the tubes via which they pass through the female’s body during laying. The embryo breathes through pores that let gas move into and out of the egg. Pore shapes and sizes vary between dinosaur eggs, and pores can be distributed evenly or unevenly over the shell surface. They are usually situated between the shell units.

Nests and clutches

If biological processes rather than taphonomic processes have grouped a cluster of eggs, it is known as a clutch (Fig. 3). It is important to note that a given clutch does not correspond to a nest: a nest is the trace fossil that provides evidence of a deliberate construction made by adult dinosaurs to provide a site for incubation. Nests can have different shapes, but very few nests are known in the fossil record.

Two famous examples are those attributed to titanosaur sauropods in Auca Mahuevo, Argentina, and the theropod Troodon formosus in Egg Mountain, Montana. (Fig. 4). The eggs in them differ in shape, size and position in the nest, but both nests have an outer rim that protects the clutch from flooding and predation. The titanosaurian nests are elongated and kidney-shaped, with eggs distributed along the axis; the nesting traces of Troodon are circular to elliptical in shape, and eggs group at the centre of the nest.

The study of dinosaur nests always requires very detailed and exhaustive documentation in the field. Palaeontologists must precisely map the clutch and the nest to document how the eggs are arranged. Traditionally, studies of dinosaur-egg clutches used two-dimensional maps of eggs to assess the clutch morphology. Now, by using photogrammetric techniques and laser technology, palaeontologists can model the eggs as three-dimensional objects (such as spheroids or ellipsoids) and record their spatial position in the clutch or nest. The three-dimensional model also allows visualization of the egg positions and clutch architecture from any perspective.